1. Field of the Invention
The present invention relates generally to the field of compositions that form abrasion-resistant coatings and more particularly to such a composition that is curable at room temperature, utilizing atmospheric moisture in the cure mechanism. A feature of the coating of the invention is that its weatherability may be enhanced.
2. Description of the Prior Art
Numerous compositions for forming abrasion-resistant coatings on various types of substrates are well-known in the art. Such abrasion-resistant coatings are frequently applied over polycarbonate substrates. The coated substrates are used as a substitute for glass, as in the case of lenses for automobile head lamps. Other uses include coatings for glass, polyethylene terephthalate containers, wood surfaces and additives for inks and pigmented coatings.
Silicone compositions of the above type are disclosed in U.S. Pat. No. 4,455,205 to Olson et al. (issued Jun. 19, 1984 and assigned to General Electric Company, Schenectady, N.Y.) The coatings comprise an ultraviolet ("UV") light-curable composition prepared from a multifunctional acrylate monomer, an acryloxyfunctional silane and aqueous colloidal silica in a polar solvent. After mixing the components, the solvent and remaining water is removed in vacuo, with gentle heating, a step which is termed "stripping." A photoinitiator is then added and the composition may be cured by exposure to UV radiation.
U.S. Pat. No. 5,403,535 Blizzard et al. discloses a radiation-curable abrasion-resistant coating which utilizes a multifunctional acrylate monomer, an alkoxy-functional aminosilane, colloidal silica and acrylate-terminated polyalkylenoxide. In the composition of Blizzard et al. the alkoxy-functional aminosilane forms a Michael adduct with the multifunctional acrylate. An excess of multifunctional acrylate is used so that curing of the composition can be carried out by ultraviolet light-initiated free radical polymerization.
U.S. Pat. No. 3,986,997 Clark discloses a thermally-curable abrasion-resistant coating composition including a dispersion of colloidal silica and hydroxylated silsesquioxane in an alcohol-water medium. The coating composition is useful for providing a scratch-resistant coating on spectacles.
European Patent Application Publication No. 0 486 469 A1 discloses a thermally-curable abrasion-resistant coating composition prepared from the hydrolysis/condensation of an alkoxy-functional organometallic compound to a sol-gel and the addition of an alkoxy-functional silane. The materials are also disclosed as being useful as a paint additive.
While the compositions of the prior art can provide useful abrasion-resistant coatings over a wide variety of substrates, they all utilize a curing mechanism that involves the application of heat or exposure to radiation.
It is well-known in the art to add ultraviolet light absorbers and free radical scavengers (collectively referred to herein as "UV protectants") to coating formulations for the purpose of enhancing their weatherability. In a coating composition that cures by ultraviolet light radiation, only a limited amount of such protectants can be added without causing interference with the curing mechanism.
Thermally-curable coating compositions also have drawbacks. First of all, many substrates suffer deleterious effects from the thermal excursion necessary for curing such compositions. The same is true of many ultraviolet light protectants, which may decompose or lose their efficacy if exposed to high temperatures.
Thus, it would be highly desirable to provide an abrasion-resistant coating that could cured under ambient conditions, much like room temperature vulcanizable sealants, such as bathroom caulking materials. Likewise, it would be highly desirable to provide such a coating wherein the weatherability could be enhanced by the addition of substantial quantities of UV protectants.